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The Deque class

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Introduction

A Deque (pronounced “deck”) is a sequence of values in a contiguous buffer that grows and shrinks automatically. The name is a common abbreviation of “double-ended queue” and is used internally by Ds\Queue.

Two pointers are used to keep track of a head and a tail. The pointers can “wrap around” the end of the buffer, which avoids the need to move other values around to make room. This makes shift and unshift very fast —  something a Ds\Vector can’t compete with.

Accessing a value by index requires a translation between the index and its corresponding position in the buffer: ((head + position) % capacity).

Strengths

  • Supports array syntax (square brackets).
  • Uses less overall memory than an array for the same number of values.
  • Automatically frees allocated memory when its size drops low enough.
  • get(), set(), push(), pop(), shift(), and unshift() are all O(1).

Weaknesses

  • Capacity must be a power of 2.
  • insert() and remove() are O(n).

Class synopsis

Ds\Deque implements Ds\Sequence {
/* Constants */
const int MIN_CAPACITY = 8 ;
/* Methods */
public void allocate ( int $capacity )
public void apply ( callable $callback )
public int capacity ( void )
public void clear ( void )
public bool contains ([ mixed $...values ] )
public Ds\Deque copy ( void )
public Ds\Deque filter ([ callable $callback ] )
public mixed find ( mixed $value )
public mixed first ( void )
public mixed get ( int $index )
public void insert ( int $index [, mixed $...values ] )
public bool isEmpty ( void )
public string join ([ string $glue ] )
public mixed last ( void )
public Ds\Deque map ( callable $callback )
public Ds\Deque merge ( mixed $values )
public mixed pop ( void )
public void push ([ mixed $...values ] )
public mixed reduce ( callable $callback [, mixed $initial ] )
public mixed remove ( int $index )
public void reverse ( void )
public Ds\Deque reversed ( void )
public void rotate ( int $rotations )
public void set ( int $index , mixed $value )
public mixed shift ( void )
public Ds\Deque slice ( int $index [, int $length ] )
public void sort ([ callable $comparator ] )
public Ds\Deque sorted ([ callable $comparator ] )
public number sum ( void )
public array toArray ( void )
public void unshift ([ mixed $values ] )
}

Predefined Constants

Ds\Deque::MIN_CAPACITY

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